1. Field of the Invention
The invention relates generally to a transmission optical fiber used in optical communication systems, and more particularly to, a transmission optical fiber capable of maintaining an existing dispersion characteristic and also increasing the Raman optical gain with a wide effective cross section.
2. Description of the Prior Art
Optical communication is a communication method by which information is received and transmitted using light. In optical communication, an optical fiber transmission line is used as a transmission medium, a semiconductor laser or a light emitting diode is used as the light source, an optical detector is used as a light-receiving unit, and the like. The optical fiber has a dual cylindrical shape in which a core at the central portion is surrounded by a cladding portion. As the refractive index of the core portion is higher than the cladding portion, light is focused on the core portion and proceeds along the optical fiber. Normally, the one in which the diameter of the core is several μm is called “a single mode optical fiber”, and the one in which the diameter of the core is several dozens of μm is called “a multi-mode optical fiber”. The optical fiber is classified into a step type optical fiber, a slope type optical fiber, etc. depending on dispersion of the refractive index of the core. Generally, the core of the optical fiber has a stricture in which GeO2 is doped in SiO2, and the cladding of the optical fiber uses SiO2 intact or has a structure in which P2O5 and fluorine (F) are doped in SiO2.
The transmission optical fiber that is usually used in the optical communication system includes a standard single mode fiber, a dispersion managed fiber, a dispersion shifted fiber, a non-zero dispersion shifted fiber, and the like. These transmission optical fibers show different characteristics since they mainly represent a dispersion characteristic and a non-linear phenomenon. The standard single mode fiber that is most widely used as the transmission optical fiber has the difference in the refractive index of 0.004˜5 between the core and the cladding. Also, the standard single mode fiber has a zero dispersion characteristic at a 1.3 μm wavelength and has a value of about 17 ps/nm/km at a 1.55 μm wavelength. Further, the peak Raman gain efficiency of the standard signal mode fiber is about 0.34 W−1km−1. For reference, the Raman optical fiber for a lumped type Raman optical amplifier has a high peak Raman gain efficiency of about 6.5 W−1km−1. This is because the Raman gain is increased by adding a lot of GeO2 to the core, the refractive index of the core is increased by GeO2 and the effective cross section is reduced by reducing the radius of the core in order to satisfy a single mode condition, so that the ratio of the area against the light power that is obtained within the optical fiber is increased.
In case of DCF (dispersion compensating fiber, index difference=0.026), the peak Raman gain efficiency is about 3.12 W−1km−1. As the core radius is small and the refractive index of the core is high, the Raman gain efficiency is increased. In case of NZDSF (non-zero dispersion shifted fiber), a place where the dispersion value reaches zero is approximately a S-band region, and the dispersion value is low at a C-band region. Therefore, the NZDSF has a better characteristic than the standard single mode optical fiber in the optical transmission system using the C-band. The NZDSF has the peak Raman gain efficiency of 0.7 W−1km−1. Of other transmission optical fibers, DSF has the peak Raman gain efficiency of 0.66 and LEAF(Large effective area fiber) from the Coning company has the peak Raman gain efficiency of about 0.45 W−1km−1 in addition to other dispersion characteristics. Items to be greatly consider in designing the transmission optical fiber include maintaining a low non-linearity, low optical loss, and large effective cross section area, low dispersion characteristic, and the like. The dispersion managed fiber is an optical fiber that serves to remove the forward mixing generating upon transmission at a low dispersion region and make zero the average dispersion value felt by the signal upon transmission, in view of the dispersion characteristic. U.S. Pat. No. 6,044,191 entitled “Dispersion Managed Optical Waveguide” discloses a method by which an average dispersion is approximated to zero by varying the dispersion value form an positive value to a negative value depending on the length of the optical fiber and then periodically varying the diameter of the core in the optical fiber to perform dispersion management.
As the transmission optical fiber has a low refractive index of the core and a high effective cross section in view of the peak Raman gain efficiency, the non-linear coefficient value is very low and the Raman gain is very low. Therefore, a method by which GeO2 is added to the core has been conventionally used in order to increase the Raman gain. An increase in the refractive index of the core accompanies a decrease in the diameter of the core in order to match a single mode condition. However, this significantly increases the non-linear characteristic of the optical fiber, and largely changes the dispersion characteristic of the transmission medium. Further, as an increase in the refractive index of the core makes the numerical aperture bigger, a double Rayleigh back scattering coefficient is also increased. The light induced by the DRBS is intact amplified in the Raman pumped optical fiber and then reaches the receiving unit, so that the signal sensitivity is degraded. Therefore, an increase in the refractive index coefficient of the core in the transmission medium causes various other phenomena, which make obsolete the value as the transmission medium.